1. Field of the Invention
This invention relates to a process for manufacturing ceramic fibers consisting mainly of alumina and silica.
2. Description of the Prior Art
It is known that amorphous ceramic fibers consisting mainly of alumina (Al.sub.2 O.sub.3) and silica (SiO.sub.2) can be produced by melting a material containing 40 to 70% by weight of alumina, 30 to 60% by weight of silica and minor quantities of impurities, and blowing or spinning the molten material. It is also known that the thermal shrinkability of these ceramic fibers can be lowered if they are heated at a temperature not lower than their devitrification temperature so that the crystallization of mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) may take place in the vitreous material.
Amorphous ceramic fibers, however, lose their heat resistance and strength if they are exposed to a temperature of at least about 1,000.degree. C. for a long time, since a coarse crystal structure is formed by the growth of mullite crystals in glass, and makes the fibers brittle. Ceramic fibers containing mullite crystals have an improved resistance to thermal shrinkage, but it lasts only for a short time. As the growth of mullite crystals further proceeds, the formation of a coarse crystal structure brings about a reduction in fiber strength.
U.S. Pat. No. 3,449,137 teaches the manufacture of ceramic fibers containing about 1 to about 6% by weight of chromium oxide (Cr.sub.2 O.sub.3) in addition to alumina and silica. The process taught by this patent has been tested, but the ceramic fibers thereby obtained have not shown any appreciable improvement in physical properties. The addition of chromium oxide is not sufficient for preventing the formation of coarse crystals by recrystallization, which results in higher degrees of thermal shrinkage and embrittlement of ceramic fibers, when they are exposed to a high temperature for a long time.